1. Field of the Invention
The present invention relates to a wafer detection apparatus for detecting wafers and placements thereof contained in a wafer carrier, and more particularly, to a wafer detection apparatus suited for detecting overlapping of two wafers as well as a warp of a wafer.
2. Description of the Related Art
During the manufacture of silicon wafers, silicon wafers are subjected to a variety of processes such as cleaning, heating, CVD and photoresist coating, and thus need to be contained in a container for transportation between these processes. To permit a large number of silicon wafers or glass wafers to be contained, a container called wafer carrier is generally used. The wafer carrier has a plurality of stages formed on the inner walls of a casing thereof and wafers are received in the respective stages, so that a large number of wafers can be stacked up in such a manner as to face each other with a predetermined distance therebetween. To detect the number of wafers contained in a wafer carrier or the placements of the wafers, detecting means including a comb-type mapping sensor or detecting means utilizing movement of transmission-type photoelectric switches is used.
The comb-type mapping sensor has a plurality of reflection-type photoelectric sensing elements arranged in the form of comb teeth. In the case of using the detecting means having such a sensor, the teeth-like sensing elements are inserted into respective spaces between wafers which are stacked up so as to face each other with a predetermined distance therebetween, and detection light emitted from the light emitting elements and reflected by respective wafers is detected by the light receiving elements to thereby detect the presence/absence of wafers.
In the detecting means for detecting wafers while moving transmission-type photoelectric switches, on the other hand, transmission-type photoelectric switches are arranged on opposite sides of a wafer carrier so as to face each other and are moved vertically relative to the wafer carrier, and the presence/absence of each wafer in the wafer carrier is detected by determining whether or not the amount of light received by the photoelectric switch is greater than a fixed threshold, as disclosed in Unexamined Japanese Patent Publication (KOKAI) No. 6-85042, for example.
According to the wafer detection technique using the comb-type mapping sensor, the number of wafers and the placements thereof can be detected at a time by simply inserting the teeth-like elements into respective spaces between the wafers, as shown in FIG. 22, but if the wafer carrier is sealed up with wafers contained therein, the mapping sensor interferes with the wafer carrier, making it impossible to perform the wafer detection. Namely, with the comb-type mapping sensor, -the presence/absence of wafers cannot be detected from outside of the wafer carrier, and to detect wafers by means of the comb-type mapping sensor, it is necessary that part of the wafer carrier, for example, a cover thereof, should be opened to allow the teeth-like sensing elements to be inserted from the opening into respective spaces between the wafers.
According to this detection method, therefore, wafers cannot be kept in a sealed state and thus the cleanliness thereof cannot be maintained at high level throughout the manufacture of wafers, with the result that the yield of wafers lowers.
Also, in this detection method, the presence/absence of each wafer is detected based on light reflected from the peripheral edge portion of the wafer; accordingly, when two wafers are overlapped one upon the other inside the carrier, the presence of the wafers alone can be detected and such overlapping of two wafers cannot be detected. Similarly, when a wafer is received slantingly inside the carrier, only the presence of the wafer can be detected and such inclination of the wafer cannot be detected. Further, even if a wafer is warped, such a warp fails to be detected, though the presence of the wafer can be detected.
If a wafer is placed slantingly, moreover, the teeth-like elements of the mapping sensor may come into contact with the peripheral edge portion of the inclined wafer during the wafer detection, possibly damaging or breaking the wafer.
Furthermore, the mapping sensor has an extremely complicated structure and is expensive, and since the presence/absence of wafers cannot be detected properly if only one of the photoelectric sensing elements of the mapping sensor fails, the expensive sensor must be replaced with new one each time such fault occurs.
In the detecting means in which transmission-type photoelectric switches are moved for wafer detection, detection light can be emitted from outside the wafer carrier toward a wafer and received by the light receiving section also located outside the carrier, and it is therefore unnecessary to open part of the wafer carrier when the presence/absence of wafers is to be detected. Namely, the presence/absence of wafers can be detected with the wafers sealed in the wafer carrier and thus the cleanliness of the wafers can be maintained. However, this detecting means simply judges that a wafer is present if the amount of light received by the photoelectric sensor is greater than a fixed threshold; therefore, in cases where wafers are detected from outside the carrier, a flaw or a parting line on the surface of the wafer carrier can be erroneously detected as a wafer.
Further, with this detection technique in which the amount of received light is compared with a fixed threshold, even when a wafer is placed slantingly or two wafers are overlapped one upon the other, it is judged simply that a wafer is present, and such improper wafer placement cannot be detected. Similarly, even if a wafer is warped, it is only judged that a wafer is present, and the warp of the wafer fails to be detected.